Photodiodes are diodes that are capable, when properly biased, of generating hole-electron pairs when exposed to light, which can result in generation of either a current or voltage. For example, a photocurrent can be generated when light is absorbed in the depleted region of this type of junction semiconductor. An applied reverse bias voltage, to a photodiode, effectively increases the width of the photodiode's depletion junction, producing an increased responsivity, a decrease in junction capacitance, and a very linear response. Some diodes that are used specifically as photodiodes are fabricated using a PIN junction, rather than a p-n junction, to increase speed of the diode's response. The magnitude of the generated photocurrent is proportional to the light intensity and also depends on the frequency of the light.
In contrast, in the so-called photovoltaic mode of a photodiode, there is zero bias of the photodiode, so that, when the photodiode is illuminated, there is a very restricted flow of current out of the diode and a voltage build-up, to exploit the photovoltaic effect. This enables the photodiode operating in this mode to be a source of power (thus photovoltaic mode operation is useful when photodiodes are used as part of solar cells).
Applications of photodiodes, whether operating in photoconductive or photovoltaic mode, include, but are not limited to, devices such as cameras, light detectors, CT scanners, x-ray devices, safety equipment such as smoke detectors, x-ray machines at airports, security systems (e.g., airport x-ray systems), energy systems (e.g., solar cells), industry applications (e.g., bar code scanners, remote controls), military systems (e.g., night vision systems), and many other applications, as those of skill in the art will appreciate.